1. Field of the Invention
The present invention relates to a brightness level conversion apparatus, and more particularly, to a brightness control apparatus for stretching the level of brightness in the dark and bright regions of an image.
2. Description of the Related Art
Generally, when a video image signal is processed, the brightness in the image signals is frequently unbalanced or distorted. This imbalance and distortion is mainly due to varying factors such as lightings, photographing conditions, and features of a video display device. In many occasions, the brightness of the same image signal varies according to the type of video display device which reproduces the image signal. For example, when the same image signal is input and reproduced through different video display devices, the image signal can have different brightness levels in the low and high brightness regions according to characteristics of the video display device displaying the image signal. In order to control such differences in brightness, a video display device generally has an Automatic Gain Control (AGC) therein. The AGC can be automatically operated, or may be manually controlled by a user, to increase or decrease image brightness.
FIG. 1 is a block diagram for showing a conventional brightness control apparatus having an AGC.
A brightness control apparatus shown in FIG. 1 has a brightness level detector 10, an AGC 20, and a mapper 30.
The brightness level detector 10 detects the level of brightness which is used to display the low and high brightness regions of an input image signal. Using the detected brightness level, the brightness level detector 10 detects excessively low brightness in the low brightness region (generally 0˜60 levels), and/or excessively high brightness in the high brightness region (generally 180˜255 levels). The brightness level detector then outputs the detected results to the AGC 20.
If the brightness level detector 10 detects an image signal with a brightness that is too high to display in the low brightness region, the AGC 20 calculates a mapping function to decrease the brightness. If the brightness level detector 10 detects an image signal with a brightness that is too low to display in the high brightness region, the AGC 20 calculates a mapping function to increase the brightness.
The mapper 30 controls the brightness of the input image signal according to the mapping functions calculated in the AGC 20.
FIG. 2A and FIG. 2B are views for illustrating the operation of the AGC 20 shown in FIG. 1.
FIG. 2A shows a mapping function decreasing an excessively high brightness level in a low brightness region. As shown in FIG. 2A, the mapping function indicates a reduction in a brightness gain, which decreases the brightness of the low brightness region of an input image signal. FIG. 2B shows a mapping function increasing the brightness gain, which increases an excessively low brightness in a high brightness region.
FIG. 3A to FIG. 3C show the results of normalizing a probability distribution function (PDF), a cumulative distribution function based on the PDF, and a cumulative distribution function with respect to the automatic gain control results by FIG. 2A and FIG. 2B.
As is well known in the art, a histogram shows a distribution of brightness values of respective pixels which form an input image signal. A cumulative distribution function (CDF) is a function obtained by converting cumulative PDFs into a monotonic increasing function. Normalizing refers to the conversion of a cumulative distribution function into a relation of output brightness values with respect to input brightness values.
FIG. 3A shows a PDF of an image signal input to the brightness level detector 10. The PDF classifies the brightness of an input image signal into levels 0–255 and shows the number of pixels of each brightness level. FIG. 3B shows the result of a conversion of the PDF of FIG. 3A into a cumulative distribution function. When the input image signal has a resolution of 720×480, the input image signal has a final cumulative value of 345,600 pixels when all the PDF are accounted for. FIG. 3C shows the result of normalizing the cumulative distribution function shown in FIG. 3B. Here, the normalization result has output the brightness values corresponding to the brightness values of the input pixels.
FIG. 4A shows a PDF for an input image signal based on the brightness gain control of FIG. 2B. As shown in FIG. 4A, the number of pixels is increased in the high brightness value region A. FIG. 4B shows a result of the conversion of the PDF of FIG. 4A into a cumulative distribution function. Here, the slope of the cumulative distribution function is greatly increased in region B due to the increase in the number of pixels in the high brightness region of the PDF. FIG. 4C shows a result of normalizing the cumulative distribution function of FIG. 4B. The normalization result is obtained by converting the cumulative distribution function of FIG. 4B into a relation between input brightness and output brightness. As shown in FIG. 4C, a region C has the maximum brightness value of 255. That is, when an input image signal is mapped in and output from the mapper 30, the overall brightness of the image signal increases.
The conventional brightness value control apparatus described above has a problem in that it brightens the overall screen to unnecessary levels when the brightness gain is increased. Also, the problem of degrading contrast ratios is present since the brightness values of the pixels in the brightened region have little or no difference between them.